The difference between the different time settings isn't that complicated.

In a normal game, the game ends at turn 500 if no civilization manages to meet any of the other victory requirements until then. In an epic game, it's turn 1000, and in marathon it's 2000. You'll notice that in no matter what mode you play, the game will move faster in time at the start than the end. One thing to note is that in addition to there being more turns, everything costs more. The time costs, in terms of years, is roughly the same between the modes, but it takes more turns, more production or more gold in Epic and Marathon. The same is true for research, so if it cost 10 turns to research in standard mode, it's roughly 40 turns in marthon. Another thing to note is that unit movement isn't slowed down, so while producing an army takes much longer than in other modes, invading another civilization shouldn't take much longer. You should still be careful when attacking since the cost of losinig units in marathon is higher than in the standard game(4 times as long to build new ones).

Reduced to a sentence, the difference between the time settings is simply: Time takes four times longer to pass, everything costs four times more.

So, what you're saying is that this is the only good Tomb Raider game since the first one?